Multiple sclerosis, an immune-mediated disease of the central nervous system, is characterized by the impaired function of regulatory cells that fail to suppress self-reactive effector cells. We have previously found that polysaccharide A, a capsular antigen derived from the human gut commensal Bacteroides fragilis, can induce a population of regulatory T cells. Herein, we demonstrate that naïve T cells isolated from patients with multiple sclerosis have the capacity to acquire regulatory characteristics when stimulated in vitro with polysaccharide A. This study demonstrates the amplification of a regulatory T cell response by a gut-derived commensal antigen in those with multiple sclerosis.